In the past, a variety of ultrasonic instruments have been devised for the detection and localization of fishes. Usually, pulse echo techniques have been used wherein pulses of acoustic energy are repetitively transmitted, and the presence of, and distance to, fish or other reflectors is indicated by the amplitude and time delay characteristics of returning echoes as indicated on a graphic recording medium or other visual display. Where bottom fishes are of particular interest, it is desirable to selectively display only the echoes from the depth zone immediately adjacent the bottom. Previously, a method has been available for automatically scanning and recording fish echo signals from the depth zone contiguous with the bottom, regardless of depth changes. This technique has been called seabed lock (or seabed referenced) echo sounding. However, such systems used in the past require the use of complex recording and indicating techniques and are far too expensive and fragile to be used by fishermen of modest means fishing from small open boats. Therefore the purpose of the present invention is to overcome these and other limitations of the prior art, so that, for the first time, ultrasonic fish detection apparatus with bottom zone selectivity can be made available for use by the small scale (artisanal) fishermen of the poorer developing countries.